1. Field of the Invention
This invention is related to electronic image capture. Specifically, the invention is related to reducing reset noise in image sensors.
2. Description of Related Art
Image sensor circuits are used in a variety of different types of digital image capture systems, including products such as scanners, copiers, and digital cameras. The image sensor is typically composed of an array of light-sensitive pixels that are electrically responsive to incident light reflected from an object or scene whose image is to be captured.
The performance of an image capture system depends in large part on the sensitivity of each individual pixel in the sensor array and its immunity from noise. Pixel sensitivity is defined here as being related to the ratio of a change in the pixel output voltage to the photogenerated charge in the pixel. Noise here is defined as small fluctuations in a signal that can be caused by a variety of known sources. An image sensor with increased noise immunity yields sharper, more accurate images in the presence of environmental and other noise.
Improving the sensitivity of each pixel permits a reduction in exposure time which in turn allows the capture of images at a greater rate. This allows the image capture system to capture motion in the scene. In addition to allowing greater frame rate, higher pixel sensitivity also helps detect weaker incident light to capture acceptable quality images under low light conditions.
Another way to increase pixel sensitivity is to increase the efficiency of the photodiode by changing the photodiode responsitivity characteristics. Doing so, however, can require deviating from a standard MOS integrated circuit fabrication process, thereby further increasing the cost of manufacturing the image sensor circuit.
Integrated circuit imaging devices include an array of light detecting elements interconnected to generate analog signals representative of an image illuminating the device. Within such an integrated circuit, each complementary metal oxide semiconductor (CMOS) image sensing element contained in the integrated circuit contains a photodiode or phototransistor as a light detecting element. In one example, charges collected in accordance with the intensity of light illuminating the photodiode or phototransistor. By storing charge, an analog signal is thus generated having a magnitude approximately proportional to the intensity of light illuminating the light detecting element.
In operation, a photo-sensitive diode is first reset by placing a charge across the photodiode. Then, the photodiode is exposed to incident light which causes the charge stored on the photodiode to be dissipated in proportion to the intensity of the incident light. After a predetermined time period during which the photodiode is exposed to the incident light and charge is allowed to dissipate from the diode (i.e., the "integration" time), the amount of charge stored on the photodiode is transferred to a capacitor by opening a switch (i.e., a "SAMPLE" transistor), between the photodiode and the capacitor.
When the time arrives to read-out the charge on the capacitor, an ADDRESS is selected. After the charge on the capacitor has been read-out, the photodiode is reset by asserting a RESET signal to a reset transistor and the reset potential which is distributed across the photodiode is read-out. The amount of incident light which is detected by the photodiode is computed by subtracting the voltage that is transferred from the capacitor from the reset voltage level on the photodiode.
When determining the amount of light detected by the photodiode, noise that is generated by the switching of the reset transistor is captured during the reset of the photodiode. In addition, due to fluctuations in the power supply voltage, the reset level varies between resets. Thus, the "noise" present in the power supply also affects the reset level. It is desirable to be able to eliminate the noise which is generated by the reset of the photodiode.
It is to be noted that although a specific architecture has been provided to describe the deficiencies in the prior art, architectures which have not been described can contain the same deficiencies. Thus, the problems described above can occur in all circuits that uses a different reset level from the level at which the photodiode begins to discharge.
It is therefore desirable to have a method of using current pixel designs to achieve improved sensitivity and noise performance using electrical circuitry available with standard MOS fabrication processes.